1. Field of the Invention
The invention relates to a process for the production of rare earth metals and rare earths-containing alloys. This process is characterized in that rare earth halides and/or rare earth oxides are reduced individually, or in a mixture, in an electric arc furnace. The process utilizes one or several alkaline earth metals and, optionally, alloying additives selected from the group of iron metals and other alloying elements. Also, optionally, additions of alkali salts and/or alkaline earth metal salts can be utilized. The process takes place in an atmosphere which is largely inert to rare earth metals, rare earth compounds and alkaline earth metals. In addition, a strong agitation effect is produced by electromagnetic forces in the melt in the furnace so as to achieve reduction as quickly and as completely as possible.
2. Description of the Prior Art
Rare earth metals and rare earth alloys are used in many industrial fields. The mixed cerium metal, for example, is widely used as a metallurgical additive for steels, cast iron and magnesium, etc. Such a mixed cerium metal is a mixture made up of metals of the so-called "light rare earths" or cerite earths and which has the approximate distribution of the elements La, Ce, Pr, Nd, Sm, and Eu as found in natural deposits (bastnasites and monazites). In steel, mixed cerium metal bonds with the residual sulfur to produce a very low sulfur content steel. In cast iron, it promotes the formation of spherical graphite and reduces the porosity. In magnesium, mixed cerium metal increases the strength and resistance to heat.
The oldest and perhaps most widely known application of mixed cerium metal is its use in the production of ignition alloys. Such alloys are based on an alloy of mixed cerium metal with iron as well as various other metals that enhance the pyrophoric property, producibility and storability. Alloy types such as LaNi.sub.5, in which La may be replaced in part by Ce, Pr and Nd, and Ni by Co, Cr, Cu, Fe, are capable of storing hydrogen with formation of rare earth hydrides. In the sixties, Strnat found the YCo.sub.5 - and Y.sub.2 Co.sub.17 -compounds have a very high uniaxial magnetic crystal anisotropy and thus possess hard magnetic properties. These findings led to the development of a number of rare earth alloys with 3d-transition elements of type SEA5 or SE2A17. The cerite earths were used in these rare earth alloys as rare earth metals, in particular Sm, and Co as 3d-transition metal, in part substituted with Fe, Mn, Cr, and also Cu. Excellent hard magnetic properties, i.e., high energy products, remanence and high coercive field strengths are the common characteristics of this type of rare earth alloy. Permanent magnets have been industrially manufactured on the basis of SmCo.sub.5 and Sm.sub.2 Co.sub.17 since the seventies.
In the years thereafter, the discovery of Strnat has led to intensive worldwide research activities resulting in enhanced hard magnetic materials based on rare earths. The hard magnetic alloys based on neodymium, iron and boron, developed in the laboratories of General Motors and Sumitomo Special Metals, at about the same time, represent at least for now, the limit of these developments. The European patent applications of General motors (European applications Nos. 0 108 474 A2 and 0 125 752 A2) and Sumitomo Special Materials (European applications Nos. 0 101 552 A2; 0 106 948 A2; 0 125 347 A2; 0 126 179 A1; and 0 126 802 A1) describe such alloys, their manufacture and processing to hard magnetic materials. Viewing the applications filed by these two laboratories in combination, it is obvious that they overlap one another with respect to the composition of the alloy. A significant difference, however, exists in the preparation of the alloy for the production of hard magnetic materials. According to the Sumitomo process, the alloys structured from their individual components are preferably melted in an induction furnace, cast in blocks, subsequently crushed and ground to particles in the .mu.m-range. For achieving anisotropic magnets, the resulting powder is compressed in a magnetic field, to form blanks, and sintered. The sintered blanks are then subjected to a heat treatment, whereupon final magnetizing is carried out.
According to the General Motors process, the alloy is formed in the normal manner from its individual components and then is melted and rapidly cooled by casting on a rotating copper roll (melt spinning). In this cooling step, the alloy solidifies to an extremely microcrystalline or amorphous structure. The resulting platlet-shaped powder is subsequently re-ground and then formed into magnetic materials by means of plastic or metal bonding agents. If one were to disregard the variations in the special manufacturing steps between these two processes, there are no special procedures followed for producing the actual alloys or special starting materials. Rather, conventional methods are employed. Known procedures have been introduced into this technological field, whereby preferably pure neodymium or neodymium prealloyed with iron, iron and boron or ferroboron are used as starting materials. The literature contains various methods (see Ullmann, Volumes 9 and 21) for the production of neodymium and most other rare earth metals and their iron-containing prealloys. The most widely known and used procedures are fusion electrolysis and metallothermal reduction. In the fusion electrolysis process, the halides of the rare earths are preferably used as raw materials. They are often used with alkali halides or alkaline earth halides. The rare earth metals separated on the cathode may be pure or prealloyed with metals from the iron group in the periodic system or another alloying element. In an electrolysis process developed by the Bureau of Mines, rare earth oxides are used as the raw material. The electrolyte is a mixture composed of various rare earth alkali fluorides or alkaline earth fluorides. In the metallothermal reduction process, the halides of the rare earths are usually used as the raw material. In some cases, alkali halides and alkaline earth halides are added as slag-forming agents or fluxing agents. While the alkali metals and/or alkaline earth metals may serve as a reducing agent, calcium is the preferred reducing agent. As a rule, the metallothermal reduction is carried out in a closed vessel in an atmosphere inert to both the reducing agent and the rare earth metal formed in the course of this step.
The electrolytic manufacture of different rare earth metals with the use of their halides and, in particular, chlorides, requires that the chlorides be free from bonded water and oxygen compounds (e.g., oxychlorides) that may be present. Furthermore, as a rule, only those rare earth metals whose melting point does not significantly exceed 1000.degree. C. can be produced in an economically justifiable way. Adding iron to the alloy, for example, will reduce the melting point, however, such alloying makes the electrolysis conditions more difficult and, as a rule, precludes the use of refractory metals as lining materials. European patent application No. 0 177 233 (Sumitomo) describes the electrolytic manufacture of iron-containing Nd-alloys. However, this application does not seem to teach how to produce specific high-iron boron-containing alloys with a composition conforming to one of the finished magnet alloys (NdFeB). Furthermore, the fact that only a small cathode cross section (cathode load) is possible poses problems in terms of electrolysis technology.
Furthermore, the metallothermal reduction processes using the rare earth halides or rare earth oxides as raw materials and calcium, for example, as the reducing agent produce a whole series of process engineering problems. The reaction, which takes place at a relatively slow rate and incompletely, requires that additional energy be supplied from outside the reactor. The ambient atmosphere has to be inert to the reducing agent and the reaction product. This means that the material of the crucible or lining of the reaction vessels has to meet stringent requirements. For example, no lining consisting of tantalum, molybdenum or tungsten is suitable for the production of iron-containing alloys. Linings consisting of MgO, Al.sub.2 O.sub.3 and/or CaO are required, but these put up little resistance to molten chlorides or fluorides and have an insulating effect on the heat supply. Furthermore, the process conditions usually permit only batch operation which results in high costs. The direct manufacture of a finished Se-Co magnet alloy requires a high expenditure for the necessary equipment (AT-PS No. 336,906 - Th. Goldschmidt), and only produces impure alloy powders. These powders have to be purified via chemical purification procedures to remove slag or reaction products. The advantage of the metallothermal manufacture of rare earth metals and alloys over fusion electrolysis lies in the relatively higher reaction rate. In addition, wider temperature ranges can be utilized. Also, the water and oxygen content of the raw materials used is less critical.